MI communication systems that require a user to wear a headset or body pack preferably include small aerials that use ferrite cores to achieve transmission efficiency. Driving such an aerial system typically requires the use of a transformer to obtain the strongest possible magnetic field where only a limited supply voltage is available. Conventionally, the transformer and the aerials (i.e., the two magnetic components) are separate items--the transformer is wound on a toroidal core and the aerials are wound on a cylindrical rod or solenoid.
It is typical for MI communication systems with duplex links to operate two channels--one channel for transmitting and one channel for receiving. The signal frequencies for each channel are usually only a fraction of an octave apart. Therefore, it is advantageous to use an aerial system having two separate aerials, one for transmitting and one for receiving. These aerials preferably are arranged to have near zero mutual inductance, thus avoiding the use of complex filters which can be bulky at the frequencies used for MI. To achieve near zero mutual inductance, the transmit and receive aerials are arranged to be mutually orthogonal and have coincident geometric centers.
Furthermore, portable MI communication systems usually have unequal rectangular cross-sections, as the direct effect of providing a housing around a printed circuit board. These systems preferably use a mix of cylindrical or solenoid aerials and air-cored loop aerials to accommodate these packaging limitations.
Accordingly, there is a need for small and efficient MI aerial systems in portable units having limited supply voltages. It is desired that these MI aerial systems transmit the strongest possible magnetic induction field while having a high transmission efficiency. It is further desired that these MI aerial systems include separate transmit and receive aerials having zero mutual inductance to avoid using complex filters which can be bulky at the frequencies used for MI.